1. Field of the Invention
The present invention relates to the field of underground storage tanks and, more specifically, to a secondary containment capsule for providing added leakage protection and leakage detection capabilities for underground petroleum storage tanks.
2. Prior Art
In recent years, concerns have been raised over the environmentally hazardous leakage of petroleum products from insecure underground storage tanks. Several states have adopted laws requiring new underground tanks to provide some form of secondary containment to prevent leakage from single wall tanks. Some state codes also require that such new tanks incorporate a leak detection means capable of detecting leakage of the stored product from the primary containment means into the secondary containment means or leakage of ground water or other liquids from outside the secondary containment means through the secondary container to provide advanced warning of any leakage.
One way to provide secondary containment for a primary storage tank is to form a second tank or envelope around the first tank, creating, in essence, a double walled tank. Several kinds of such double walled tanks are known in the prior art. A tank having a rigid steel or fiberglass (fiberglass reinforced plastic or FRP) inner tank and a flexible, polyethylene outer jacket is disclosed by U.S. Pat. No. 4,607,522. U.S. Pat. No. 4,561,292 discloses a tank having two fiberglass walls, the second fiberglass wall being formed by applying a fiberglass layer over a grid of plastic mats separated by fiberglass ribs. U.S. Pat. No. 4,537,328 discloses a tank with a steel inner tank and a fiberglass outer tank, the fiberglass outer tank being formed by applying fiberglass sheets over a grid of semicircular PVC pipe segments attached to the surface of the inner steel tank. Other double walled storage tank designs are disclosed by U.S. Pat. Nos. 4,568,925 and 4,523,454.
Although the double wall tanks of the prior art provide a greater degree of leakage protection than single walled tanks, they are also more difficult to manufacture and more expensive. In the prior art, the secondary tanks are custom formed directly on the primary tank, requiring the use of large fixtures for holding the primary tank while the secondary tank is being formed. Mass production of the secondary tank, separately from the primary tank, is therefore not possible.